Dagger 2 field injection在Android中不起作用

问题是,我尝试在Dagger 2中使用字段注入,但在运行时,应该注入的字段总是 null 。我还尝试使用MVVM模式。这是我的代码:

ProfileActivity。java:

@Override
protected void onStart() {
    super.onStart();
    Log.d(TAG, "ProfileActivity: onStart: ");

    final ProfileViewModel profileViewModel 
         = ViewModelProviders.of(this).get(ProfileViewModel.class);
    profileViewModel.init();
    profileViewModel.getUser().observe(this, new Observer<User>() {
        @Override
        public void onChanged(@Nullable User user) {
            if (user != null) {
                Log.d(TAG, "ProfileActivity: onStart: " + user.toString());
            } else {
                Log.d(TAG, "ProfileActivity: onStart: user == null");
            }
        }
    });
}

ProfileViewModel。java:

public class ProfileViewModel extends ViewModel {
    private LiveData<User> user;

    @Inject
    UserRepository userRepository;

    public ProfileViewModel() {
        Log.d(TAG, "ProfileViewModel: Constructor: ");
    }

    public void init() {
        Log.d(TAG, "ProfileViewModel: init: ");
        user = userRepository.getUser();
    }

    public LiveData<User> getUser() {
        Log.d(TAG, "ProfileViewModel: getUser: ");
        return user;
    }
}

用户存储库。java:

@Singleton
public class UserRepository {
    private LiveData<User> user;

    @Inject
    public UserRepository() {
        Log.d(TAG, "UserRepository: Constructor: ");
    }

    public LiveData<User> getUser() {
        Log.d(TAG, "UserRepository: getUser: ");
        if (user != null) {
            return user;
        } else {
            // There should be userDao.load() call,
            // but it had been omitted for brevity.
            MutableLiveData<User> user = new MutableLiveData<>();
            user.setValue(DB.getUser());
            return user;
        }
    }
}

我的应用程序。java:

public class MyApplication extends MultiDexApplication implements HasActivityInjector {
    @Inject
    DispatchingAndroidInjector<Activity> dispatchingAndroidInjector;

    @Override
    public void onCreate() {
        super.onCreate();
        DaggerMyApplicationComponent.create().inject(this);
    }

    @Override
    public DispatchingAndroidInjector<Activity> activityInjector() {
        return dispatchingAndroidInjector;
    }
}

MyApplicationModule。java:

@Module
public abstract class MyApplicationModule {
    @ContributesAndroidInjector
    abstract ProfileActivity contributeActivityInjector();
}

MyApplicationComponent。java:

@Component(modules = { AndroidInjectionModule.class, MyApplicationModule.class})
@Singleton
public interface MyApplicationComponent extends AndroidInjector<MyApplication> {
    void inject(ProfileActivity profileActivity);
}

在运行时,我可以看到下一个日志:

ProfileActivity: onStart:
ProfileViewModel: Constructor:
ProfileViewModel: init:

应用程序崩溃 user = userRepository.getUser(); 在…内 ProfileViewModel 的 init() 方法

这意味着 UserRepository 没有注射。也可以通过缺失来表示 UserRepository: Constructor: 日志

我的错在哪里?非常感谢。